xiedeyantu commented on code in PR #4495:
URL: https://github.com/apache/calcite/pull/4495#discussion_r2261608902
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core/src/main/java/org/apache/calcite/sql/type/ReturnTypes.java:
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@@ -227,7 +227,8 @@ public static SqlCall stripSeparator(SqlCall call) {
@Override public RelDataType
inferReturnType(SqlOperatorBinding opBinding) {
final RelDataType type = super.inferReturnType(opBinding);
- if (opBinding.getGroupCount() == 0 || opBinding.hasFilter()) {
+ if (opBinding.allowChangeNullable()
Review Comment:
This is not the direct cause. The direct cause lies in the expansion of
groupsets. For example, agg(group={0,1,2}, groups=[{0,1,2}, {}]) would, under
the original logic, be expanded into agg(group={0,1,2}, groups=[{0,1,2}]) and
agg(group={0,1,2}, groups=[{}]). However, this is incorrect. It should be
expanded into agg(group={0,1,2}, groups=[{0,1,2}]) and agg(group={},
groups=[{}]). This discrepancy is what leads to the subsequent issue.
For simple SQL statements (such as select sum(a) from t), the original logic
would infer the sum as nullable = true. However, when there is a group by (such
as select sum(a) from t group by b), the sum remains inferred as nullable =
false. There is a special case involving operations like rollup (i.e., {0,1,2},
{0,1}, {0}, {}), where the last grouping set is an empty grouping set. In this
logic, when encountering grouping, all group sets need to be fully expanded
(otherwise it’s impossible to correctly compute the grouping value for each
set), and then connected with union. According to the above logic, the sum in
the final empty grouping set will be inferred as nullable = true, which is
inconsistent with the other grouping sets where nullable = false. This
inconsistency can cause issues in subsequent rule executions (such as
ProjectMergeRule, ProjectToCalcRule, etc.). Therefore, in this special case, an
additional parameter is introduced to force certain aggregate functions to
automatically infer nullable as true. I hope this explanation is clear.
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